The invention relates to the use of surface-coated rutile modification TiO2 pigments as an anticorrosive white pigment.
TiO2 pigments that have a grain size d50 of 0.1 to 1.0 xcexcm are used to lighten dark media and in order to lend covering power to coating materials. The more the light refracting of the pigments is superior to the light refracting of the binding agent, the better suited for this are the TiO2 pigments. Coating properties are influenced by the incorporated TiO2 pigments. This behaviour, in terms of application techniques, is already adjusted during the production of the TiO2 pigments. This relates not only to the optical properties, such as scattering power, brightness and lustre, but also to further parameters, such as weather resistance, dispersability, settling behaviour and wettability.
The increasing importance of corrosion prevention and the recognition of the health risks originating from known anticorrosive pigments which contain lead and chromate have led to the development of less toxicologically harmful anticorrosive pigments based on phosphates, in particular Zn3(PO4)2, as well as molybdate pigments, tungstate pigments and zirconate pigments, metallic zinc, organic pigments and pure barrier pigments, such as micaceous iron ore.
TiO2 pigments having a corrosion-inhibiting action, the particles of which are sheathed with an outer layer of hydrated silicon dioxide, hydrated aluminium oxide or a mixture of these two components, are the subject matter of EP 0412686 B. Cations, consisting of Ca ions, Zn ions, Li ions, Ba ions, Mg ions, Sr ions, Co ions or Pb ions, which effect corrosion prevention by means of ion exchange, are chemically bonded to the coating. EP 0595341 A refers to a coating material which contains TiO2 pigments which are coated with 1 to 10% by weight of a zinc alloy and are used for corrosion prevention. CZ-B-235 851 proposes inter alia TiO2 pigments which are coated with a layer of Mn3(PO4)2 and mixed with varnishes for the purposes of corrosion prevention. CZ-B 235 887 discloses a TiO2 base body which is coated firstly with Zn phosphate, secondly with Ca phosphate and/or Mg phosphate and thirdly with Mn phosphate and/or Al phosphate, and is used for the purposes of corrosion prevention.
Finally, the use of a surface-coated TiO2 base body, preferably in the rutile modification and having a grain size of 0.1 to 1.0 xcexcm, as an anticorrosive white pigment, with firstly manganese phosphate and secondly aluminium oxides and/or aluminium hydroxides being precipitated onto the TiO2 base bodies, is described in WO 99/65994. The disadvantage in this connection is the yellow cast of these anticorrosive white pigments.
An object of the present invention is to overcome the disadvantages of the prior art and in particular to make available anticorrosive white pigments that have no yellow cast and at the same time have at least the same or a greater anticorrosive effect than previously known anticorrosive white pigments.
This object is achieved by means of the use of surface-coated rutile modification TiO2 pigments having a grain size d50 of the primary particles of 0.1 to 1.0 xcexcm as an anticorrosive white pigment, with firstly magnesium phosphate and secondly aluminium oxides and/or aluminium hydroxides being precipitated onto the TiO2 pigments.
It is of advantage that a greater anticorrosive effect is achieved in comparison with the anticorrosive white pigments described in CZ-B-235 887 and in WO 99/65994, without the anticorrosive white pigments having a yellow cast.
In a further development of the invention, magnesium phosphate is partially replaced by zinc phosphate.
The substances precipitated onto the TiO2 pigments are preferably precipitated in the following proportions in relation to the quantity of TiO2 used (data in % by weight):
It is particularly preferred that the substances precipitated onto the TiO2 pigments are precipitated in the following proportions in relation to the quantity of TiO2 used (data in % by weight):
A method for preparing the surface-coated pigments that are to be used in accordance with the invention can be carried out as follows:
A suspension is prepared from rutile modification TiO2 having a primary particle grain size d50 of 0.1 to 1.0 xcexcm, and water. The suspension preferably contains 200 to 350 g TiO2 per liter of suspension. The suspension is raised to a preferred temperature of 60 to 90xc2x0 C. whilst being stirred. The pH-value of the suspension is preferably adjusted to a value of 6 to 7.5. An alkali hydroxide or a mineral acid, preferably sulphuric acid, can be used to adjust the pH-value.
One or more solutions containing Mg2+ and one or more solutions containing phosphate ions, preferably phosphoric acid, are added to the TiO2-suspension. Magnesium phosphate is thereby precipitated onto the TiO2-particles. The pH-value in this precipitation reaction is preferably adjusted to values between 6 and 7.5. Subsequently, one or more solutions containing Al3+ are added, with the pH-value preferably being adjusted to values between 6 and 7.5. Aluminium oxides and/or aluminium hydroxides are thereby precipitated onto the TiO2-particles.
The suspension that is obtained is preferably stirred further for 1 to 5 hours (preferred temperature 60 to 90xc2x0 C.) and the TiO2 pigments that are provided with a layer of firstly magnesium phosphate and secondly aluminium oxide and/or aluminium hydroxide are filtered off in the usual way, dried and, if applicable, ground.
The pigments thus obtained can be used as anticorrosive white pigments. Examples of such applications are the automobile sector (electro-dip coatings), household appliances and the field of industrial varnishes. The latter includes the use of the pigments in primers, band coatings (coil and can coatings), single-layer varnish coatings (for example for the DIY market) and electro-dip coatings.
The subject matter of the invention is explained in greater detail with the aid of the following examples: